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Code C0055

Tiny
KEVBOAC
  • MEMBER
  • 2008 CHEVROLET EXPRESS
  • 6.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 121,000 MILES
While driving the vehicle, the customer experienced the ABS and brake lights came on, vehicle started to drive very sluggishly. Limped home then towed to shop. Code indicates rear wheel speed sensor malfunction. Vehicle does not have rear wheel ABS nor speed sensors on rear wheels or on the pumpkin. On tech 2 rear wheel speed is erratic, spiking at times, fronts are stable. We replaced the ECBM and programmed to vehicle no change, replaced the transmission output sensor no change. Replaced the ECM no change. Running out of ideas?
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Wednesday, February 9th, 2022 AT 7:45 PM

11 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Take a look at this testing. Under circuit testing it is telling you to test for the vehicle speed sensor circuit.

You are correct that this does not have rear ABS, but it calculates this via the vehicle speed sensor signal.

So, we need to remove the connector and the EBCM and see if it is above 10 volts on the yellow/black wire.

You may have a wiring issue, so we need to run through this testing and let us know what you find.

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if you have questions. Thanks
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Friday, February 11th, 2022 AT 8:47 AM
Tiny
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  • 6 POSTS
We scoped the input signal wire to the EBCM from the ECM and under 10 MPH it has a beautiful square wave pattern then all of a sudden it will have garbage for a short time.
We have run new wires ( layed over ) from the EBCM to the ECM the signal wire, no change.
The engine is off during this testing, vehicle is on a rack, and we are turning the tires by hand. Driving the vehicle wave pattern is great after 10 MPH, never bobbles or has static.
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Friday, February 11th, 2022 AT 9:34 AM
Tiny
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So, if this is on the vehicle speed signal circuit from the ECM to the EBCM (yellow/black wire) then that is a calculation of the vehicle speed and is shared with the EBCM.

So, if you are getting this pattern from the ECM to the EBCM then it is just sharing faulty info that it is receiving.

Meaning the ECM is not creating this data, it is just sharing what it is getting. That means we need to look at what is giving this info to the ECM.

That would be the vehicle speed sensor.

You said the rear speed sensor are spiking at times. What exactly where you looking at and what was spiking on this?
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Friday, February 11th, 2022 AT 5:06 PM
Tiny
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This vehicle does not have rear speed sensors, it uses the transmission output sensor. This is fed to the TCM as a sign wave, it had no noise on it we could see on the lab scope. The TCM sends the signal to the ECU on the data bus, which we have no way to read.
The ECU sends a 128K square wave to the EBCM (yellow/black wire) this is where we see the noise between the square wave patterns, in the picture I posted. We have run two new wires and built them as a twisted pair, nine wraps per foot, then wrapped in aluminum foil and attached a ground lead to it also. No change. Code sets with 40 ft of vehicle movement.

Code set of C0055-0F rear speed sensor erratic operation.
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Saturday, February 12th, 2022 AT 2:24 PM
Tiny
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Okay. I understand. I was clarifying when you said this in the first post, "On tech 2 rear wheel speed is erratic, spiking at times, fronts are stable."

However, I think we are on the same page so let's just move on.

I think you are confirming that the code is correct because this code is telling you that there is an issue with the vehicle speed sensor circuit because you have the code and then see the faulty pattern on the scope.

So, if it is not wiring, you replaced both modules, then the only thing I can think is there is something that the sensor is picking up from the speed sensor tone wheel. Have you removed the sensor and used a borescope to inspect the teeth? I attached the info that we talked about with the operation and then the tone wheel teeth.

Maybe one is cracked or has debris on it giving a faulty signal when it passes by.
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Saturday, February 12th, 2022 AT 8:28 PM
Tiny
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Yes, we have inspected the tone ring, removed the drive shaft to check for end play, found none, tried to pry the tone ring side to side and left and right no movement, no debris, no cracks.
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Monday, February 14th, 2022 AT 9:00 AM
Tiny
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Okay. This is an odd one. So, either we are missing something or making an assumption that is wrong that is taking us in the wrong direction.

So, before we start backtracking have you scoped the pattern from the speed sensor to the ECM? Or did you just do the pattern from the ECM to the EBCM?

Basically, I suspect the ECM is just sending out the pattern it is getting so I know you wired from the ECM to the EBCM but if the sensor is new and the tone wheel is fine, then we need to rule out the wiring from the sensor to the ECM.
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Monday, February 14th, 2022 AT 1:56 PM
Tiny
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Okay. So, I have more info. When driving the vehicle slowly, 3-5 MPH, the scan tool shows rear speed sensor at 3, 4, then will jump up to 16 or 43 MPH then come back down. Additionally, the customer complained he was driving down the highway at 65+ and all of a sudden lost power and would not go over 25 MPH, pulled off road and had towed to us. The only code we found when we scanned it was C0055, no p codes, no transmission codes, nothing. We have not replaced the TCM. We have replaced the EBCM, ECM, Both Sensors on transmission.
Think we should try the TCM? That is where the sensors both feed directly?
Thanks
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Wednesday, February 16th, 2022 AT 12:12 PM
Tiny
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Wow. That is great info. I have no doubt about what is happening now.

Basically, this code is setting because the ECM is giving the ECBM faulty info. However, it is just passing along the info that it is given because as you said, the TCM gets the info directly from the sensor.

So, the sensor sends the signal to the TCM, the TCM sends it to the ECM. The ECM then sends the info to the EBCM via the communication network.

So, if the sensor is good and not sending a faulty signal but the ECM is sending the faulty signal, we have an issue between the sensor and ECM. That would be wiring or the TCM.

See the diagram below.

So, the question that is left, what is causing that faulty signal? TCM would be a likely cause.
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Wednesday, February 16th, 2022 AT 5:34 PM
Tiny
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So, we changed the TCM with OEM TCM and programmed, all issues gone, no noise and no ABS, C0055 set. Thanks for the info and help.
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Thursday, February 17th, 2022 AT 4:31 PM
Tiny
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Wow. Clearly not the first thing you think of, but it makes total sense and great to know that is what it was.

Thanks again as I am sure this post will help others in the future because you were very thorough in your testing and description.

Please come back to 2CarPros next time.
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Thursday, February 17th, 2022 AT 4:34 PM

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